Redroot Pigweed (Amaranthus retroflexus) is a dicot weed in the Amaranthaceae family. In Ontario this weed first evolved resistance to Group C3/6 herbicides in 2005 and infests Corn (maize). Group C3/6 herbicides are known as PSII inhibitors (Nitriles) (Inhibition of photosynthesis at photosystem II). Research has shown that these particular biotypes are resistant to bentazon, and bromoxynil and they may be cross-resistant to other Group C3/6 herbicides.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

Greenhouse trials comparing a known susceptible Redroot Pigweed biotype with this Redroot Pigweed biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetics

Genetic studies on Group C3/6 resistant Redroot Pigweed have not been reported to the site. There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature

Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database. If you know anything about the mechanism of resistance for this biotype then please update the database.

Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes. If you have any information pertaining to the fitness of Group C3/6 resistant Redroot Pigweed from Ontario please update the database.

The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Ontario have been instrumental in providing you this information. Particular thanks is given to Francois Tardif for providing detailed information.

Experiments were conducted to confirm imazethapyr resistance in redroot amaranth (Amaranthus retroflexus L.) and study the target-site based mechanism for the resistance. Whole-plant response experiments revealed that the resistant (R) population exhibited 19.16 fold resistance to imazethapyr compared with the susceptible (S) population. In vitro ALS activity assay demonstrated that the imazethapyr I50 value of the R population was 21.33 times greater than that of the S population. However, qRT-PCR analysis revealed that there is no difference in ALS gene expression between the R and S populations. Sequence analysis revealed an Asp-376-Glu substitution in ALS in the R population. In order to verify that the imazethapyr resistance was conferred by Asp-376-Glu mutation, the ALS-R and ALS-S genes were fused to the CaMV 35S promoter and introduced into Arabidopsis respectively. The expression of ALS-R in transgenic Arabidopsis plants exhibited 13.79 fold resistance to imazethapyr compared to ALS-S transgenic Arabidopsis..

Three putative resistant Amaranthus retroflexus L. populations were collected in Heilongjiang province in China. Whole plant bioassays indicated high resistance (RI > 10) to imazethapyr in the three populations. In vitro acetolactate synthase (ALS) assays revealed that ALS from populations H3, H17 and H39 was less sensitive to imazethapyr inhibition compared to the susceptible population H76. The half-maximal inhibitory concentration (I50) values for H3, H17 and H39 were 14.83, 15.27 and 268 times greater, respectively, than that of the susceptible population H76. Three nucleotide mutations resulted in three known resistance-endowing amino acid substitutions, Ala-205-Val, Trp-574-Leu and Ser-653-Thr in the three resistant populations respectively. Therefore, ALS target-site mutations in resistant A.retroflexus could be responsible for imazethapyr resistance..

Nowadays, both worldwide and in Serbia, for weed eradication in orchards mostly herbicides based on glyphosate, glufosinate-ammonium, diquat and others are used. Intensive glyphosate application has led to the development of resistant weed species, which has consequently resulted in a decrease in its effectiveness. In our country, areas under orchards amount to 224.000 hectares, which certainly points to a significant herbicide use and a possibility that weed resistant populations have developed. For this reason, seeds of several weed species from areas where glyphosate has been intensively used for years were collected (localities: Indjija, Brestovac, Šabac, Vršac, Sombor, Glogonjski Rit, Padinska Skela and Surčin). Plants were grown in controlled conditions and in the open field. Plant material was then crushed using liquid nitrogen, and the extraction of shikimic acid was performed using hydrochloric acid (1 g of plant material+5 ml 1M HCl). 24 hours later the amount of shikimic acid was detected using high-pressure liquid chromatography. The analysis of the obtained results showed that species Amaranthus retroflexus (loc. Šabac), Abutilon teophrasti (loc. Brestovac) and wild Helianthus annuus (loc. G. Rit) have developed a certain degree of glyphosate resistance..

Results are presented of trials conducted in 2012 in Italy to prove the existence of amaranth (Amaranthus retroflexus) resistant to herbicide imazamox and thifensulfuron-methyl, and to investigate the efficacy of some herbicides to control the weed in soyabean. Some suggestions to manage the herbicide resistance problem are also given..

A previously unknown glyphosate resistance mechanism, amplification of the 5-enolpyruvyl shikimate-3-phosphate synthase gene, was recently reported in Amaranthus palmeri. This evolved mechanism could introgress to other weedy Amaranthus species through interspecific hybridization, representing an avenue for acquisition of a novel adaptive trait. The objective of this study was to evaluate the potential for this glyphosate resistance trait to transfer via pollen from A. palmeri to five other weedy Amaranthus species (Amaranthus hybridus, Amaranthus powellii, Amaranthus retroflexus, Amaranthus spinosus, and Amaranthus tuberculatus). Field and greenhouse crosses were conducted using glyphosate-resistant male A. palmeri as pollen donors and the other Amaranthus species as pollen recipients. Hybridization between A. palmeri and A. spinosus occurred with frequencies in the field studies ranging from <0.01% to 0.4%, and 1.4% in greenhouse crosses. A majority of the A. spinosus × A. palmeri hybrids grown to flowering were monoecious and produced viable seed. Hybridization occurred in the field study between A. palmeri and A. tuberculatus (<0.2%), and between A. palmeri and A. hybridus (<0.01%). This is the first documentation of hybridization between A. palmeri and both A. spinosus and A. hybridus..